(i) Field of the Invention
The present invention relates to a plasma jet torch equipped with a constrained chip through whose nozzle a plasma generated by gas discharge is jetted, and the constrained chip is made of a metal, is forcedly cooled and functions as one discharge electrode. More specifically, it relates to a material and a shape of the constrained chip mounted on a tip of plasma jet torch.
(ii) Description of the Prior Art
Plasma cutting techniques can be classified into a plasma arc cutting method by which a metal can be cut, and a plasma jet cutting method by which a metal and a nonmetal can be cut. As shown in FIG. 1, the plasma jet cutting method uses an apparatus in which a plasma jet power source 14 and a high-frequency generator 16 are connected between a tungsten electrode 10 and a water-cooked constrained chip 12 made of oxygen-free copper. The cutting operation of the plasma jet cutting method will be carried out as follows: In the first place, a working gas (e.g., an argon gas) is caused to flow between the tungsten electrode 10 and the constrained chip 12, and the high-frequency generator 16 is then operated to discharge a high-frequency arc, thereby destroying the electric insulation of the working gas and forming an ionized electric passage. An arc current continuously flows from the plasma jet power source 14 through the working gas, and at this time, a plasma flow 20 is generated by arc heat. This heat of the plasma flow 20 is utilized to cut an article 22 to be cut.
According to this method, the plasma flow 20 is jetted through the nozzle of the constrained chip 12 by the thermal expansion of the plasma gas itself. Thus, the arc current does not flow through the article 22 to be cut, in contrast to the plasma arc cutting method. For this reason, the plasma jet cutting method also permits the cutting of non-conductive materials (e.g., fire bricks and concretes). However, an energy density of the generated high-temperature plasma flow rapidly declines, as the plasma flow jetted through the nozzle leaves the nozzle, and therefore the thermal efficiency of the plasma jet cutting method is as low as 10 to 20%. In consequence, this cutting method cannot be considered to be an efficient cutting technique. Accordingly, the plasma jet cutting method has not usually been put to practical use, and as a commercially available product, only a 30A grade torch for cutting acrylic plates has been present.
However, articles to be cut are not always made of conductive materials. For example, in a decommissioning operation of nuclear fuel facilities, the plasma jet cutting method is considered to be promising, because it is capable of cutting various materials which form the constitutional equipments of the facilities such as concrete and plastics in addition to metals. Thus, an experimental plasma Jet cutting torch was made for the purpose of verifying the principle of this method. The torch for the principle verification comprised a commercially available plasma arc torch additionally provided with an outer nozzle serving as an anode, in which the plasma flow could be generated between a tungsten electrode and the outer nozzle. It was confirmed that a metal (a material: SUS-304) having a thickness of 25 mm and a refractory brick (an electrocast brick) having a thickness of 150 mm could be pierced and crushed with the stable plasma flow obtained by this cutting torch (the plasma Jet cutting torch was held on an article to be cut for a certain time to achieve the piercing) (in the case that compressive strength was about 2,000 kg/cm2, they were crushed by thermal shock), and so the plasma Jet cutting torch was practical as a cutting device for the decommissioning.
However, the experimental torch for the principle verification was large (length=about 500) and heavy (weight=about 4 kg), and so it was difficult to handle the torch together with a remote manipulation device such as an MS (a master slave), a manipulator or a robot. Thus, taking the remote manipulation into consideration, a practical torch for generating the plasma flow between a tungsten electrode and a water-cooling constrained copper chip was made so that the shape and weight of the torch might have the same shape and weight as in a plasma arc cutting torch (length=about 240 mm and weight=about 1 kg) which was utilized together with the MS manipulator or the like. A bore diameter and a constrained nozzle portion of the constrained chip were set to 3 mm and 3 mm, respectively.
However, in this miniaturized practical torch, the constrained chip which was an anode was noticeably molten, so that any stable plasma flow could not be obtained and the cutting was impossible. Hence, it was apparent that the experimental cutting torch was impractical as the cutting device for the decommissioning of nuclear fuel facilities.